[PATCH v29 18/32] mm: Add guard pages around a shadow stack.

Yu-cheng Yu
2021-08-20T18:20:54Z

INCSSP(Q/D) increments shadow stack pointer and 'pops and discards' the
first and the last elements in the range, effectively touches those memory
areas.

The maximum moving distance by INCSSPQ is 255 * 8 = 2040 bytes and
255 * 4 = 1020 bytes by INCSSPD. Both ranges are far from PAGE_SIZE.
Thus, putting a gap page on both ends of a shadow stack prevents INCSSP,
CALL, and RET from going beyond.

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
Reviewed-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Kees Cook <keescook@chromium.org>
---
v25:
- Move SHADOW_STACK_GUARD_GAP to arch/x86/mm/mmap.c.

v24:
- Instead changing vm_*_gap(), create x86-specific versions.
---
arch/x86/include/asm/page_types.h | 7 +++++
arch/x86/mm/mmap.c | 46 +++++++++++++++++++++++++++++++
include/linux/mm.h | 4 +++
3 files changed, 57 insertions(+)

diff --git a/arch/x86/include/asm/page_types.h b/arch/x86/include/asm/page_types.h
index a506a411474d..e1533fdc08b4 100644
--- a/arch/x86/include/asm/page_types.h
+++ b/arch/x86/include/asm/page_types.h
@@ -73,6 +73,13 @@ bool pfn_range_is_mapped(unsigned long start_pfn, unsigned long end_pfn);

extern void initmem_init(void);

+#define vm_start_gap vm_start_gap
+struct vm_area_struct;
+extern unsigned long vm_start_gap(struct vm_area_struct *vma);
+
+#define vm_end_gap vm_end_gap
+extern unsigned long vm_end_gap(struct vm_area_struct *vma);
+
#endif /* !__ASSEMBLY__ */

#endif /* _ASM_X86_PAGE_DEFS_H */
diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
index f3f52c5e2fd6..81f9325084d3 100644
--- a/arch/x86/mm/mmap.c
+++ b/arch/x86/mm/mmap.c
@@ -250,3 +250,49 @@ bool pfn_modify_allowed(unsigned long pfn, pgprot_t prot)
return false;
return true;
}
+
+/*
+ * Shadow stack pointer is moved by CALL, RET, and INCSSP(Q/D). INCSSPQ
+ * moves shadow stack pointer up to 255 * 8 = ~2 KB (~1KB for INCSSPD) and
+ * touches the first and the last element in the range, which triggers a
+ * page fault if the range is not in a shadow stack. Because of this,
+ * creating 4-KB guard pages around a shadow stack prevents these
+ * instructions from going beyond.
+ */
+#define SHADOW_STACK_GUARD_GAP PAGE_SIZE
+
+unsigned long vm_start_gap(struct vm_area_struct *vma)
+{
+ unsigned long vm_start = vma->vm_start;
+ unsigned long gap = 0;
+
+ if (vma->vm_flags & VM_GROWSDOWN)
+ gap = stack_guard_gap;
+ else if (vma->vm_flags & VM_SHADOW_STACK)
+ gap = SHADOW_STACK_GUARD_GAP;
+
+ if (gap != 0) {
+ vm_start -= gap;
+ if (vm_start > vma->vm_start)
+ vm_start = 0;
+ }
+ return vm_start;
+}
+
+unsigned long vm_end_gap(struct vm_area_struct *vma)
+{
+ unsigned long vm_end = vma->vm_end;
+ unsigned long gap = 0;
+
+ if (vma->vm_flags & VM_GROWSUP)
+ gap = stack_guard_gap;
+ else if (vma->vm_flags & VM_SHADOW_STACK)
+ gap = SHADOW_STACK_GUARD_GAP;
+
+ if (gap != 0) {
+ vm_end += gap;
+ if (vm_end < vma->vm_end)
+ vm_end = -PAGE_SIZE;
+ }
+ return vm_end;
+}
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 4548f75cef14..354f38d21eed 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2729,6 +2729,7 @@ struct vm_area_struct *vma_lookup(struct mm_struct *mm, unsigned long addr)
return vma;
}

+#ifndef vm_start_gap
static inline unsigned long vm_start_gap(struct vm_area_struct *vma)
{
unsigned long vm_start = vma->vm_start;
@@ -2740,7 +2741,9 @@ static inline unsigned long vm_start_gap(struct vm_area_struct *vma)
}
return vm_start;
}
+#endif

+#ifndef vm_end_gap
static inline unsigned long vm_end_gap(struct vm_area_struct *vma)
{
unsigned long vm_end = vma->vm_end;
@@ -2752,6 +2755,7 @@ static inline unsigned long vm_end_gap(struct vm_area_struct *vma)
}
return vm_end;
}
+#endif

static inline unsigned long vma_pages(struct vm_area_struct *vma)
{
--
2.21.0